📚 Gene Expression: A-Level CIE Biology Key Points | 基因表达 考点精讲
Gene expression is the fundamental process by which the genetic code stored in DNA is decoded to produce functional products, typically proteins. Understanding this multi-step pathway is essential for A-Level CIE Biology, as it connects molecular genetics with phenotypic traits and forms the basis for topics like mutations, gene regulation and genetic engineering. In this revision guide, we walk through the key stages, from transcription to translation, and explore the control mechanisms that ensure genes are expressed at the right time and in the right cell.
基因表达是将储存在DNA中的遗传密码解码并产生功能性产物（通常是蛋白质）的基本过程。理解这多步骤通路对A-Level CIE生物考试至关重要，因为它将分子遗传学与表型性状联系起来，并构成突变、基因调控和基因工程等主题的基础。在这份考点精讲中，我们将梳理从转录到翻译的关键阶段，并探讨确保基因在正确时间和正确细胞中表达的控制机制。
1. Overview of Gene Expression | 基因表达概述
The central dogma of molecular biology describes the flow of genetic information: DNA is transcribed into mRNA, which is then translated into a polypeptide. This sequence-based transfer of information is universal, though some viruses use reverse transcription (RNA → DNA). In both prokaryotic and eukaryotic cells, gene expression involves transcription in the nucleus (or nucleoid region in prokaryotes) and translation on ribosomes in the cytoplasm. However, the processes are more compartmentalised and subject to additional regulatory steps in eukaryotes.
分子生物学的中心法则描述了遗传信息的流向：DNA转录为mRNA，然后mRNA翻译为多肽。这种基于序列的信息传递是通用的，尽管某些病毒使用逆转录（RNA → DNA）。在原核和真核细胞中，基因表达都包括在细胞核（或原核的类核区）进行的转录以及在细胞质核糖体上进行的翻译。但在真核生物中，这些过程更加区域化，并且受到额外的调控步骤的影响。
A key concept tested in exams is that not all genes are expressed in every cell; differential gene expression allows specialisation. For instance, the insulin gene is transcribed in pancreatic beta cells but silenced in skin cells. This is controlled by transcription factors, promoters and epigenetic modifications.
考试中经常出现的一个关键概念是：并不是所有基因都在每个细胞中表达；基因的差异表达使得细胞特化成为可能。例如，胰岛素基因在胰岛β细胞中被转录，但在皮肤细胞中被沉默。这是由转录因子、启动子和表观遗传修饰来控制的。
2. The Genetic Code | 遗传密码
The genetic code is a triplet code: a sequence of three nucleotides (a codon) on mRNA specifies one amino acid. The code is read in a non-overlapping, commaless manner from a fixed start point. There are 64 possible codons but only 20 standard amino acids, meaning the code is degenerate – multiple codons can code for the same amino acid, which reduces the impact of point mutations.
遗传密码是一种三联体密码：mRNA上三个核苷酸组成的序列（一个密码子）决定一个氨基酸。密码子以不重叠、无逗号的方式从固定的起点开始读取。总共有64种可能的密码子，但标准氨基酸只有20种，这意味着密码子具有简并性——多个密码子可以编码同一种氨基酸，这降低了点突变的影响。
The start codon is AUG, which codes for methionine in eukaryotes (or formylmethionine in prokaryotes) and signals the beginning of translation. Three stop codons – UAA, UAG and UGA – do not code for any amino acid; they cause termination of the polypeptide chain. The genetic code is nearly universal, with few exceptions (e.g. mitochondrial DNA), which supports the idea of a common evolutionary origin.
起始密码子是AUG，它在真核生物中编码甲硫氨酸（在原核生物中编码甲酰甲硫氨酸），并标志着翻译的开始。三个终止密码子——UAA、UAG和UGA——不编码任何氨基酸；它们导致多肽链合成的终止。遗传密码几乎是通用的，只有少数例外（例如线粒体DNA），这支持了共同进化起源的观点。
3. Transcription: From DNA to mRNA | 转录：从DNA到mRNA
Transcription is the synthesis of a single-stranded mRNA molecule complementary to the template strand of DNA. The enzyme RNA polymerase binds to the promoter region upstream of a gene. In prokaryotes, the sigma factor helps RNA polymerase recognise the promoter; in eukaryotes, a set of general transcription factors is required. DNA unwinds, and RNA polymerase moves along the template strand in the 3′ → 5′ direction, building the mRNA in the 5′ → 3′ direction. Free ribonucleoside triphosphates (ATP, UTP, GTP, CTP) pair with exposed bases – adenine pairs with uracil (in RNA), cytosine with guanine.
转录是指合成一条与DNA模板链互补的单链mRNA分子。RNA聚合酶结合在基因上游的启动子区域。在原核生物中，σ因子帮助RNA聚合酶识别启动子；在真核生物中，则需要一套通用转录因子。DNA解旋，RNA聚合酶沿模板链的3′ → 5’方向移动，以5′ → 3’方向合成mRNA。游离的核糖核苷三磷酸（ATP、UTP、GTP、CTP）与暴露的碱基配对——腺嘌呤与尿嘧啶配对（在RNA中），胞嘧啶与鸟嘌呤配对。
Transcription continues until RNA polymerase encounters a terminator sequence. In prokaryotes, this often involves a hairpin loop structure that causes the polymerase to dissociate. In eukaryotes, termination is coupled to the cleavage and polyadenylation of the pre-mRNA. The immediate product in eukaryotes is pre-mRNA, which must be processed before translation.
转录持续进行，直到RNA聚合酶遇到终止子序列。在原核生物中，这通常涉及一个发夹环结构，导致聚合酶脱离。在真核生物中，终止与前mRNA的切割和聚腺苷酸化偶联。真核生物中的直接产物是前mRNA，必须经过加工才能进行翻译。
4. Post-Transcriptional Modifications in Eukaryotes | 真核生物的转录后修饰
In eukaryotic cells, the primary transcript (pre-mRNA) undergoes three major processing steps before it becomes a mature mRNA. First, a 5′ cap (a modified guanine nucleotide) is added, which protects the mRNA from degradation and helps ribosome binding during translation. Second, a poly-A tail – a string of about 50–250 adenine nucleotides – is added to the 3′ end, which also enhances stability and facilitates export from the nucleus. Third, splicing removes non-coding introns and joins together the coding exons. This is carried out by a spliceosome, a complex of small nuclear ribonucleoproteins (snRNPs).
在真核细胞中，初级转录本（前mRNA）在成为成熟mRNA之前要经历三个主要的加工步骤。首先，添加一个5’帽（一个修饰过的鸟嘌呤核苷酸），它可以保护mRNA免于降解，并有助于翻译时核糖体的结合。第二，在3’端添加一条由大约50-250个腺嘌呤核苷酸组成的多聚腺苷酸尾（poly-A tail），这也增强了稳定性并促进mRNA从细胞核输出。第三，剪接去除非编码的内含子，并将编码的外显子连接起来。这一过程由剪接体（一种由小核糖核蛋白颗粒snRNP组成的复合体）完成。
Alternative splicing allows a single gene to code for multiple protein isoforms by combining different sets of exons. This greatly increases the diversity of the proteome without expanding genome size. Examination questions often ask students to identify exons and introns from a given DNA sequence and predict the mature mRNA.
可变剪接通过组合不同的外显子组，使一个基因能够编码多种蛋白质异构体。这大大增加了蛋白质组的多样性，而无需扩大基因组规模。考试题常要求学生从给定的DNA序列中识别外显子和内含子，并预测成熟的mRNA序列。
5. Translation: mRNA to Protein | 翻译：mRNA到蛋白质
Translation converts the nucleotide language of mRNA into the amino acid sequence of a polypeptide. It takes place on ribosomes, which consist of a large and a small subunit made of rRNA and proteins. The process can be divided into initiation, elongation and termination. Initiation begins when the small ribosomal subunit binds to the mRNA near the 5′ cap and scans for the start codon AUG. A special initiator tRNA carrying methionine (or fMet) pairs with the start codon, and the large subunit joins, forming a functional ribosome with the initiator tRNA in the P site.
翻译将mRNA的核苷酸语言转换为多肽的氨基酸序列。它在核糖体上进行，核糖体由rRNA和蛋白质组成的大、小亚基构成。该过程可分为起始、延伸和终止。当小亚基结合到mRNA 5’帽附近并扫描寻找起始密码子AUG时，翻译起始开始。一种携带甲硫氨酸（或甲酰甲硫氨酸）的特殊起始tRNA与起始密码子配对，然后大亚基结合，形成一个功能性核糖体，起始tRNA位于P位点。
During elongation, a new aminoacyl-tRNA enters the A site, and its anticodon base-pairs with the mRNA codon. The ribosome catalyses the formation of a peptide bond between the amino acid in the P site and the incoming amino acid. The ribosome then translocates, moving the tRNAs to the E and P sites, and the process repeats. Elongation continues until a stop codon enters the A site. Release factors bind, prompting hydrolysis of the peptidyl-tRNA bond and releasing the completed polypeptide.
在延伸过程中，一个新的氨酰tRNA进入A位点，其反密码子与mRNA密码子碱基配对。核糖体催化P位点上的氨基酸与新进入的氨基酸之间形成肽键。然后核糖体移位，将tRNA分别移至E位点和P位点，循环往复。当终止密码子进入A位点时，延伸停止。释放因子结合，促使肽基-tRNA键水解，并释放完整的多肽链。
Multiple ribosomes can translate a single mRNA simultaneously, forming a polysome. This boosts the rate of protein synthesis and is a common examination diagram to interpret.
多个核糖体可以同时翻译一条mRNA，形成多聚核糖体。这提高了蛋白质合成速率，也是考试中常见的需要解读的图示。
6. The Role of tRNA and Ribosomes | tRNA和核糖体的作用
Transfer RNA (tRNA) molecules are adapters that link the genetic code to specific amino acids. Each tRNA has a cloverleaf secondary structure and folds into an L-shaped tertiary structure. At the 3′ end, there is an amino acid attachment site (always CCA) where the cognate amino acid is covalently attached by aminoacyl-tRNA synthetase. The anticodon loop contains a triplet of bases (the anticodon) that is complementary to an mRNA codon. Each tRNA is specific to one amino acid, but because of wobble base pairing, some tRNAs can recognise more than one codon.
转运RNA（tRNA）分子是连接遗传密码与特定氨基酸的适配器。每个tRNA具有三叶草形的二级结构，并折叠成L形的三级结构。在3’端有一个氨基酸结合位点（总是CCA序列），同源的氨基酸由氨酰tRNA合成酶共价连接在此。反密码子环含有一个碱基三联体（反密码子），与mRNA密码子互补。每种tRNA对应一种特定的氨基酸，但由于摇摆碱基配对，某些tRNA可以识别多个密码子。
Ribosomes provide the platform for translation. The prokaryotic 70S ribosome (composed of 50S and 30S subunits) and the eukaryotic 80S ribosome (60S and 40S) differ slightly, but both harbour three binding sites for tRNA: the A (aminoacyl), P (peptidyl) and E (exit) sites. The ribosome’s peptidyl transferase activity (derived from rRNA, making it a ribozyme) catalyses peptide bond formation.
核糖体为翻译提供了平台。原核生物的70S核糖体（由50S和30S亚基组成）和真核生物的80S核糖体（60S和40S亚基）略有不同，但两者都有三个tRNA结合位点：A位点（氨酰位）、P位点（肽基位）和E位点（出口位）。核糖体的肽基转移酶活性（来源于rRNA，这使其成为核酶）催化肽键的形成。
7. Mutations and Their Effects on Gene Expression | 突变及其对基因表达的影响
A gene mutation is a change in the nucleotide sequence of DNA. Substitution mutations replace one base with another; they can be silent (no change in amino acid due to code degeneracy), missense (a different amino acid, e.g. sickle cell anaemia – GAG to GTG causing Glu → Val in the beta-globin chain), or nonsense (introduction of a premature stop codon). Insertion or deletion mutations cause a frameshift if the number of nucleotides added or removed is not a multiple of three, altering the entire downstream reading frame and usually producing a non-functional protein.
基因突变是指DNA核苷酸序列的改变。置换突变是用一个碱基替换另一个碱基；它们可以是沉默突变（由于密码子简并性，氨基酸序列不变），错义突变（氨基酸改变，例如镰刀型细胞贫血症——GAG突变为GTG导致β-珠蛋白链中谷氨酸变为缬氨酸），或无义突变（引入了提前终止密码子）。插入或缺失突变如果插入或缺失的核苷酸数不是3的倍数，就会引起移码突变，改变下游整个阅读框，通常产生无功能的蛋白质。
The effect of a mutation on gene expression depends on its location. A mutation in the promoter can reduce or abolish transcription by preventing RNA polymerase binding. A mutation in a splice site can result in aberrant mRNA processing. Mutations in regulatory genes (e.g. lacI) can cause constitutive expression. In CIE exams, you may be asked to predict the consequence of a given mutation on the polypeptide sequence, which requires using the genetic code table.
突变对基因表达的影响取决于其位置。启动子区域的突变可以通过阻止RNA聚合酶结合来降低或消除转录。剪接位点的突变可能导致异常的mRNA加工。调控基因（如lacI）的突变可以引起组成型表达。在CIE考试中，你可能需要根据给定的突变预测其对多肽序列的影响，这需要运用遗传密码表。
8. Gene Regulation: The lac Operon in Prokaryotes | 基因调控：原核生物的乳糖操纵子
The lac operon in E. coli is a classic model of prokaryotic gene regulation. It consists of a promoter (P), an operator (O), and three structural genes: lacZ (β-galactosidase, which hydrolyses lactose), lacY (permease, for lactose uptake) and lacA (transacetylase). A separate regulatory gene, lacI, codes for the lac repressor protein. When lactose is absent, the repressor binds tightly to the operator, physically blocking RNA polymerase from transcribing the structural genes – the operon is off.
大肠杆菌的乳糖操纵子是原核生物基因调控的经典模型。它包括一个启动子（P）、一个操纵基因（O）以及三个结构基因：lacZ（β-半乳糖苷酶，水解乳糖）、lacY（通透酶，负责乳糖摄取）和lacA（转乙酰酶）。一个单独的调控基因lacI编码乳糖阻遏蛋白。当没有乳糖时，阻遏蛋白紧密地结合在操纵基因上，从物理上阻止RNA聚合酶转录结构基因——操纵子关闭。
When lactose is present, it is converted to allolactose, which acts as an inducer. Allolactose binds to the repressor, causing a conformational change that prevents it from binding to the operator. RNA polymerase can then transcribe the genes, and the enzymes for lactose metabolism are produced. This is an example of inducible gene expression. However, even in the presence of lactose, glucose can inhibit expression via catabolite repression.
当乳糖存在时，它被转化为异构乳糖，后者充当诱导物。异构乳糖与阻遏蛋白结合，使其发生构象变化，无法再与操纵基因结合。RNA聚合酶得以转录这些基因，并产生乳糖代谢所需的酶。这是可诱导基因表达的一个例子。然而，即使有乳糖存在，葡萄糖也可以通过分解代谢物阻遏作用来抑制表达。
Catabolite repression involves the CAP (catabolite activator protein) and cAMP. When glucose levels are low, cAMP accumulates and binds to CAP. The cAMP-CAP complex binds to a site near the lac promoter, enhancing the affinity of RNA polymerase for the promoter and dramatically increasing transcription. The table below summarises the operon states.
分解代谢物阻遏涉及CAP（分解代谢物激活蛋白）和cAMP。当葡萄糖水平较低时，cAMP积累并与CAP结合。cAMP-CAP复合物结合在lac启动子附近的位点，增强RNA聚合酶对启动子的亲和力，大幅提高转录水平。下表总结了操纵子的状态。
| Condition | lac Repressor | CAP-cAMP | Transcription Level |
|---|---|---|---|
| No lactose, glucose present | Bound to operator | Not bound | Very low (off) |
| Lactose, glucose present | Not bound (inactivated by allolactose) | Not bound (low cAMP) | Low |
| Lactose, no glucose | Not bound | Bound | High (fully on) |
9. Eukaryotic Gene Regulation | 真核生物基因调控
Eukaryotic gene expression is regulated at multiple levels, but transcriptional control is the most important. Promoter regions contain specific sequences such as the TATA box, where general transcription factors and RNA polymerase II assemble to form the transcription initiation complex. In addition, distal control elements called enhancers can be bound by activator proteins (specific transcription factors) to dramatically increase transcription rates. Silencers are sequences that repress transcription when bound by repressor proteins.
真核生物基因表达在多个水平受到调控，但转录调控最为重要。启动子区域含有特定序列，如TATA盒，通用转录因子和RNA聚合酶II在此组装形成转录起始复合物。此外，称为增强子的远距离调控元件可以被激活蛋白（特异性转录因子）结合，从而大幅提高转录速率。沉默子则是被阻遏蛋白结合时抑制转录的序列。
Epigenetic factors such as DNA methylation and histone modification can alter chromatin structure without changing the DNA sequence. Methylation of promoter regions usually represses
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